As an indicator for showing a biological state, the glycation degrees of various kinds of proteins are measured. Among them, a glycation degree of hemoglobin (Hb) in a blood cell, in particular, HbA1c is used as an important indicator in diagnoses, treatments and the like for diabetes, because the HbA1c reflects histories of an in-vivo blood glucose level. The HbA1c has a structure in which a glucose is bonded to a β chain N-terminal amino acid (valine) of HbA (α2β2), and its value is represented by a ratio (proportion or %) of a HbA1c amount with respect to a total Hb amount.
HbA1c is measured, for example, by a high-performance liquid chromatography (HPLC) method, an immunization method, an enzymatic method, and an electrophoresis method. Recently, the establishment of an easy-to-use measurement by the enzymatic method has been studied. An example of the method of measuring HbA1c by the enzymatic method is as follows. First, Hb is treated with protease and a fragment containing a β chain N-terminal valine is cleaved. Then, fructosyl amine oxidase (hereinafter, referred to as “FAOD”) is allowed to act on a glycated part of the fragment (i.e., a glycated part of the β chain N-terminal valine), thereby generating hydrogen peroxide. The amount of this hydrogen peroxide corresponds to a glycation amount of the β chain N-terminal valine of the Hb. Then, peroxidase (hereinafter, referred to as “POD”) and a chromogenic substrate that develops color by oxidation are added further to this reaction solution, so that a redox reaction occurs between the hydrogen peroxide and the chromogenic substrate with the POD as a catalyst. Thereafter, a chromogenic level of the chromogenic substrate is measured, for example, by an absorbance measurement. In this method, the level of the absorbance corresponds to an amount of colored chromogenic substrate, the amount of the colored chromogenic substrate corresponds to an amount of generated hydrogen peroxide, and the amount of the hydrogen peroxide corresponds to the amount of glycation as described above. In other words, the glycation amount can be measured indirectly by measuring the chromogenic level of the chromogenic substrate through such redox reaction. Further, HbA1c (%) can be calculated from this glycation amount and a total Hb amount.
This kind of measurement of HbA1c (%) often is performed by an inspection agency. Particularly in a case of medical examination, etc., normally, Hb-containing samples (for example, a whole blood sample, a blood cell sample collected from whole blood, a Hb sample collected from the whole blood) collected from patients are not subjected to a measurement right after collection. In general, these Hb-containing samples are subjected to the measurement after storage at room temperature, or in a refrigerated or frozen condition.